Kinetic Properties of the .alpha.2.beta.1 and .alpha.2.beta.2 Isoenzymes of the Na,K-ATPase

The presence of multiple isoforms of the alpha and beta subunits of the Na,K-ATPase in most mammalian tissues has hindered the understanding of the roles of the individual isoforms in directing Na,K-ATPase function. Expression of the Na,K-ATPase subunits in insect cells using recombinant baculoviruses has proven to be a useful system for the study of Na,K-ATPase function. Using this system, we have expressed the rat Na,K-ATPase alpha 2 beta 1 and alpha 2 beta 2 isoforms in Sf-9 insect cells, a cell line derived from the ovary of the fall armyworm, Spodoptera frugiperda. Both beta 1 and beta 2 isoforms can efficiently assemble with the alpha 2 subunit to produce catalytically competent Na,K-ATPase molecules. The analysis of the kinetic properties of both isozymes showed that alpha 2 beta 1 and alpha 2 beta 2 have equivalent sensitivities to ouabain, and similar turnover numbers and apparent affinities for K+ and ATP. The dependence on Na+, however, differs between the isozymes, with alpha 2 beta 2 displaying a slightly higher apparent affinity for the cation than alpha 2 beta 1. In addition, the even greater kinetic differences between Na,K-ATPase isozymes varying in alpha isoforms may be important in further differentiating the enzyme. Thus, when compared to the rat alpha 1 beta 1 Na,K-ATPase expressed in Sf-9 cells, the alpha 2 beta 1 and alpha 2 beta 2 isozymes have a lower apparent affinity for K+ and a higher affinity for Na+ and ATP.(ABSTRACT TRUNCATED AT 250 WORDS)